Do Underweight Newborns Make for Shy Adults?

New research suggests a potential connection

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WEDNESDAY, July 9, 2008 (HealthDay News) -- Being born extremely underweight may be a risk factor for a shy, cautious personality as an adult.

A study published in the July issue of Pediatrics finds that young adults who were born at extremely low birth weights were more likely to be shy, cautious and risk-averse than their counterparts who had been born at normal weights.

And that shy personality style may increase the odds of loneliness and decrease the odds of emotional well-being, heightening the risk for emotional problems, said the Canadian study authors.

But the finding between low birth weight and a retiring personality doesn't necessarily imply a cause-and-effect relationship.

"We were showing that there are these differences that seem to exist between these two groups," said Louis A. Schmidt, lead author of the study and a professor of psychology, neuroscience and behavior at McMaster University in Hamilton, Ontario, Canada. "That gives us room to say, 'Hey, there are different ways in which to become shy, one of which is early-life events.'"

Schmidt stressed, however, that researchers can't yet point to an actual cause-and-effect relationship.

"They [the study authors] didn't address why the children have low birth weight," said Dr. Jane Ripperger-Suhler, assistant professor of psychiatry and behavioral science at Texas A&M Health Science Center College of Medicine and a psychiatrist with Scott & White Mental Health Center in Temple, Texas. "They missed an opportunity to be really clear. Just because you were tiny at birth does not mean you are going to be anxious. There is something that ties those together."

Previous research has found that children born with extremely low birth weights have a higher risk of learning disabilities and emotional problems, including neuroticism, attention deficit/hyperactivity disorder and a tendency toward caution.

Relatively little research, however, has looked at whether these issues persist into adulthood.

The authors of the new study looked at 71 young adults who had been born extremely underweight, as well as 83 young adults who had been normal weight at birth.

The adults who had been extremely underweight at birth reported being shyer, having more behavioral inhibition, lower sociability, higher risk-aversion and a greater tendency to follow social convention. The results were the same for both men and women.

There were also differences within the extremely-low-birth-weight group, with lower birth weight linked to more shyness, behavioral inhibition and loneliness, among other traits.

But the study authors did provide one note of caution: The adults who had been born extremely underweight in this group also had similar relationships with peers, partners and family as the normal-birth-weight group.